This invention relates generally to medical imaging systems, and more particularly, to image reconstruction using Positron Emission Tomography (PET) systems.
A PET system generates images that represent the distribution of positron-emitting nuclides within the body of a patient. When a positron interacts with an electron by annihilation, the entire mass of the positron-electron pair is converted into two 511 keV photons. The photons are emitted in opposite directions along a line of response. The annihilation photons are detected by detectors that are placed along the line of response on a detector ring. When these photons arrive and are detected at the detector elements at the same time, this is referred to as coincidence. An image is then generated, based on the acquired image data that includes the annihilation photon detection information.
In a PET system, the image quality depends on image statistics. The image statistics may be improved by acquiring the image data for longer durations. However, the total time required to acquire the image data is limited by the decay of the radioactive isotope used in the imaging process and by the inability of the patients to remain immobile for extended durations.
Image quality may be improved by including time-of-flight (TOF) information of the emission data. Strictly speaking, TOF is the time taken by an annihilation photon to travel from the origin of annihilation to detector elements along the line of response, but this cannot be measured directly since the time at which the emission takes place is not known. Therefore, TOF usually refers to the difference in the time at which the photons are detected by the detector elements. The timing difference is used to localize the source of emission along the line joining two detector elements in TOF PET systems.
In order to maintain a good signal-to-noise ratio in the images in the reconstruction process in TOF PET systems, it is important to measure the timing difference accurately. A systematic error or bias in the estimation of the timing difference between photon detection in the two detector elements will translate into a shift of the estimated source of photon annihilation along the line joining two detector elements. Calibration errors can result in such a timing bias. For example, if the timing bias is 0.1 ns, the reconstruction process shifts the data along the line between the detector elements by 1.5 cm. Reconstruction of images with these timing biases will result in image noise, particularly if the timing bias is large compared to the timing resolution of the detectors.
In known PET systems, timing errors are measured by introducing a known source distribution into the scan region of the PET system and thereby acquiring a TOF data set. Since the location of the activity is known, the expected timing difference data may be computed, for example, if the activity is known to be at the midpoint of the line between two detectors, the expected timing difference is equal to zero. If the average measured timing data is not equal to the expected timing difference, the difference between the average measured timing data and the expected timing difference represents the TOF error for that detector element pair.
In known TOF PET systems, the photon arrival time is digitized and coincidence is determined by comparing the digital time stamps of each measured photon. In an effort to compensate for timing errors measured in the PET system, the detector signals are tuned or calibrated by introducing a variable amount of delay in the signal before it is digitized in each detector element. However, a system with a large number of detector elements will, for reasons of economy of manufacture, use a common set of electronics to process a group of detector elements. It is commonly not known which detector within the group has received a photon, and therefore which value of the delay should be applied to the signal before the signal is digitized and processed. Therefore, differences in the timing bias among the detectors in a group cannot be individually corrected, and if these differences are significant there will be a loss of accuracy in the reconstructed image.
Another known method of calibrating a TOF PET system is to introduce a delay after the signal is digitized, by changing the digital time stamp. In this event, the timing bias of a particular detector element is adjusted by changing the digital time stamp according to the least significant bit (LSB) of the digital circuit. As a result, the timing signal of each individual detector element can only be adjusted in multiples of the LSB, even though the system may have the capability to determine that the timing is a fraction of the LSB. The adjustment can reduce the timing bias to only ±½ LSB. Because this occurs for each of the two photons in the coincidence pair, the timing bias of the coincidence pair may be as large as ±1 LSB. For example, if the LSB is 50 ps, it corresponds to a location error of up to ±1.5 cm along the coincidence line of response.
Moreover, the timing biases of a plurality of detector element pairs are not stable, for example, due to time, temperature variations affecting the electronic components, and the like. Therefore, the timing biases measured during the calibration procedure may not be accurately applied to subsequent acquired scan data, resulting in degradation of the reconstructed image.